1. Field of the Invention
The present invention relates to a lighting device that has a planar light emitting face, and to a liquid crystal display device using this lighting device.
2. Description of the Related Art
Liquid crystal displays are widely used in portable devices such as notebook personal computers, cellular phones, PDAs, and electronic dictionaries. Liquid crystal displays need to place a planar lighting device behind a liquid crystal panel because the liquid crystal panel is not self-luminous. Lighting devices for portable devices are required to be thin and, at the same time, to have high luminance to make a displayed image viewable outdoors during daylight hours. Uniform light emission is also requested of lighting devices for portable devices so that a displayed image has an even brightness. As a prospective solution, sidelight type lighting devices are beginning to be researched. Sidelight type lighting devices which have a light source placed by the light emission face can be made thin overall (see JP 11-250714 A and JP 11-232918 A, for example).
FIG. 12 is a schematic vertical sectional view of this type of known conventional lighting device 50. FIG. 13 is a schematic top view of the lighting device 50. The lighting device 50 includes a light source 53, a light guiding plate 51, and a reflecting plate 54. The light source 53 is placed to the side of the light guiding plate 51, and light from the light source 53 is introduced into the light guiding plate 51. A face of the light guiding plate 51 that is opposed to a light exit face of the light guiding plate 51 (hereinafter referred to as opposed face) has a plurality of reflecting parts 52 formed in, for example, V-shaped grooves. Light trapped between the light exit face and opposed face of the light guiding plate 51 is reflected by the slopes of the V-shaped grooves of the reflecting parts 52. In short, the slopes function as reflecting surfaces. The slopes of the V-shaped grooves are set suitably so that light trapped within the light guiding plate 51 is taken out from the light exit face of the light guiding plate 51. The reflecting plate 54 reflects light that has leaked from the opposed face of the light guiding plate 51 back to the light guiding plate 51, thereby preventing the intensity of illumination light taken out from the top face of the light guiding plate 51 from lowering.
As illustrated in FIG. 13, a large number of reflecting parts 52 are arranged in arc pattern with the light source 53 as the center of the arc. The V-shaped grooves of the reflecting parts 52 have, when viewed from above, a rectangular shape, and are formed such that the longitudinal direction of the rectangle is substantially at right angles with the direction of the light source 53. Arranging the reflecting parts 52 in this manner causes reflected light that is reflected at the reflecting surfaces of the V-shaped grooves to converge in a direction perpendicular to the light exit face of the light guiding plate 51. With the reflected light thus directed in the perpendicular direction, a device to which the lighting device 50 is applied, for example, a liquid crystal display device, can display bright images.
FIG. 14 is a partial sectional view of the light guiding plate 51. The reflecting parts 52 in the form of V-shaped grooves are formed on the opposed face of the light guiding plate 51. Incident light 57b which enters from the direction of the light source 53 is reflected upward by a reflecting part 52a, which is positioned on the near side of the incident light 57b. However, when the reflecting parts 52 are arranged as illustrated in FIG. 13, a reflecting part 52b, which is positioned behind the reflecting part 52a with respect to the incident light 57b, is in the shadow of the reflecting part 52a. This causes unevenness in luminance of illumination light that exits from the light exit face of the light guiding plate 51 and, as a result, the obtained illumination light does not have a uniform intensity.